The present invention generally relates to an automatic implantable atrial defibrillator for delivering cardioverting or defibrillating electrical energy to the atria of a human heart. The present invention is more particularly directed to a crosspoint switch for use in an automatic implantable atrial defibrillator which provides cardioverting or defibrillating electrical energy. The invention also provides improved discharge control when supplying defibrillating electrical energy having a biphasic waveform to the atria of the heart.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators has become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has been the cause of these defibrillators from becoming a commercial reality. Two such defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
An implantable defibrillator must be powered by a portable, depletable power source, such as a battery. It has been long believed that as much electrical energy is required to cardiovert or defibrillate the atria of the heart as is required to cardiovert or defibrillate the ventricles of the heart, on the order of ten joules or more. In addition, episodes of atrial fibrillation occur much more frequently than do episodes of ventricular fibrillation. As a result, due to the contemplated required cardioverting or defibrillating energy levels for cardioverting or defibrillating the atria of the heart and the predicted required frequency of delivering such energies, it has long been believed that an implantable atrial defibrillator would deplete its power source so rapidly that frequent battery replacement would be required. Since battery replacement would require the surgical implanting of the defibrillator, it has long been believed that an implantable atrial defibrillator could not be a commercial reality. To this day, a commercially implantable atrial defibrillator remains unavailable.
Defibrillators generally include a means, such as a storage capacitor, for storing the electrical energy required to cardiovert or defibrillate the heart. A charging circuit is provided for charging the storage capacitor to a potential of several hundred volts. Control circuitry is further provided to detect the level of charge stored on the storage capacitor and to control the discharge of the capacitor through lead means to the heart.
Implantable defibrillators are known in the art which deliver cardioverting or defibrillating electrical energy from the storage capacitor with a biphasic waveform. With such a biphasic waveform, the cardioverting or defibrillating electrical energy is initially applied with a first polarity, and then with a second and reversed polarity in rapid succession. The electrical energy is applied through lead means having at least two electrodes in electrical contact with the heart.
A crosspoint switch is generally utilized for supplying the biphasic electrical energy from a single storage capacitor. The crosspoint switch may be configured to first couple a positive terminal of the storage capacitor to a first electrode and a negative terminal of the storage capacitor to a second electrode during a first cardioverting phase of the biphasic waveform and to couple the positive terminal of the storage capacitor to the second electrode and the negative terminal of the storage capacitor to a first electrode during a second cardioverting phase of the biphasic waveform Crosspoint switches exhibit a distinct advantage because they conserve space within an implantable defibrillator by eliminating the need for a second storage capacitor with reverse polarity and its associated charging circuit.
One concern in implementing a crosspoint switch for delivering cardioverting electrical energy with a biphasic waveform is inadvertently shorting out the storage capacitor between the first cardioverting phase and the second cardioverting phase. This can occur during switching if both terminals of the storage capacitor are coupled to the same electrode, even briefly. If this occurs, the shorted capacitor could lose all and most assuredly a significant portion of its stored electrical energy.
To reduce the risk of shorting out the storage capacitor, crosspoint switches have been associated with control circuits which verify that the connections within the switch for the first cardioverting phase are broken before completing the connections for the second cardioverting phase. To provide this verification, such control circuits have had to be rather elaborate to introduce switching delays to prevent the storage capacitor from being shorted out.
In addition to being rather elaborate, prior crosspoint switch control circuits have exhibited the further disadvantage of lacking the ability to adequately control the durations of the first and second cardioverting phases. Such phase duration control is desirable for tailoring the applied electrical therapy to a patient's particular condition. Ideally, the duration of both the first and the second cardioverting phases should be independently controllable from a zero time duration (analogous to a monophasic waveform) to a predetermined maximum time duration. Previous crosspoint switch control circuits have lacked this flexibility. The crosspoint switch and control circuit of the present invention overcomes these shortcomings of earlier devices. The crosspoint switch and control circuit of the present invention provides for decoupling the storage capacitor from the lead means while the crosspoint switch connections are changed. The present invention also provides independent control of the duration of each cardioverting phase. As disclosed herein, first and second digital counters are provided for controlling the duration of the first and second cardioverting phases, respectively. Other features and advantages of the present invention shall become apparent hereinafter.